Configuring Grbl on the WriteHand

Below are the settings I entered into Grbl to get the WriteHand up and operational, and the reasons I chose them. All those not mentioned I left at the default state. For an explanation on each, check the Grbl page here.

$0 – Step pulse, microseconds

Looking at the datasheets for the A4988 stepper driver, it appears the step pulse length is rated at 1 microsecond. So I set $0 = 1. This throws an error, so I reset it to $0 = 3, which appears to be the minimum length allowable.

$3 = 3 – Direction port invert, binary

$22 = 1 – Homing Cycle, bool

$23 = 2 – Homing Direction Invert, binary

$24 = 300 – Homing feed, mm/min

$25 = 2000 – Homing seek, mm/min

$27 = 3 – Homing pull-off, mm

$100 and $101 – [X,Y] steps/mm

For a rough estimate, I calculated the steps per mm. Later I will find out the actual movement and tweak this number. To calculate the steps/mm, I measured the diameter of the timing belt around the stepper pully (~14mm) to find the circumference (~44mm). There are 200 steps in one full rotation of the stepper motor, however I am using 1/8 micro-stepping, so I multiply this by 8, then I divide this 800 microsteps by 44 to get 36.364 steps/mm. $100 = 36.364 and $101 = 36.364.

$110 and $111 – [X,Y] Max rate, mm/min

To make sure the motors can move quickly, the maximum rates should be raised quite high, say, 200 mm/sec. Converts to 12,000 mm/min. I set mine as follows: $110 = 12000, $111 = 12000.

$120 and $121 – [X,Y] Acceleration, mm/sec^2

I arbitrarily chose my initial acceleration to be 500 mm/sec^2. It has been working well, without missing steps when moving. $120 = 500, $121 = 500.

$130 and $131 – [X,Y] Max travel, mm

Using the manual stepping arrows in the Grbl Controller, I could easily find the maximum limits of each axis. My work space is roughly 300 mm x 250 mm. $130 = 300, $131 = 250.

Grbl also lets you create starting blocks. Starting blocks are g-code messages sent immediately after homing the machine. First, I set my feed rate (target movement speed) to 6000 and my spindle speed to 10 (a number 0-255 which determines the amount my servo moves to drop the pen). Second, since my machine homes to a less than optimal location for X=0 and Y=0, my machine moves to the negative direction the full length of the machine (300). G91 turns the machine to relative mode, to move -300x from where it was (0,0).